ide_fifo.v

module ide_fifo
(
	input         clk,             // bus clock
	input         clk_en,
	input         reset,           // reset 
	input  [15:0] data_in,         // data in
	output reg [15:0] data_out,    // data out
	input         rd,              // read from fifo
	input         wr,              // write to fifo
	input         packet_in,
	input         packet_out,
	input  [12:0] packet_count,
	output        packet_in_last,  // last word is read in packet_in state
	output        full,            // fifo is full
	output        empty,           // fifo is empty
	output        last_out,        // the last word of a sector is being read
	output        last_in          // the last word of a sector is being written
);

// local signals and registers
reg   [15:0] mem [4095:0]; // 16 bit wide fifo memory
reg   [12:0] inptr;        // fifo input pointer
reg   [12:0] outptr;       // fifo output pointer
wire         empty_rd;     // fifo empty flag (set immediately after reading the last word)
reg          empty_wr;     // fifo empty flag (set one clock after writting the empty fifo)
reg          rd_old;
reg          wr_old;

always @(posedge clk)
	if (clk_en) begin
		rd_old <= rd;
		wr_old <= wr;
	end

// main fifo memory (implemented using synchronous block ram)
always @(posedge clk)
	if (clk_en) begin
		if (wr)
			mem[inptr[11:0]] <= data_in;
	end

always @(posedge clk)
	if (clk_en)
		data_out <= mem[outptr[11:0]];

// fifo write pointer control
always @(posedge clk)
	if (clk_en) begin
		if (reset)
			inptr <= 0;
		else if (wr_old & ~wr)
			inptr <= inptr + 1'd1;
	end

// fifo read pointer control
always @(posedge clk)
	if (clk_en) begin
		if (reset)
			outptr <= 0;
		else if (rd_old & ~rd)
			outptr <= outptr + 1'd1;
	end

// the empty flag is set immediately after reading the last word from the fifo
assign empty_rd = inptr==outptr ? 1'b1 : 1'b0;

// after writting empty fifo the empty flag is delayed by one clock to handle ram write delay
always @(posedge clk)
	if (clk_en)
		empty_wr <= empty_rd;

assign empty = empty_rd | empty_wr;

// at least 512 bytes are in FIFO 
// this signal is activated when 512th byte is written to the empty fifo
// then it's deactivated when 512th byte is read from the fifo (hysteresis)
// special handlig of packet commands
assign full = (inptr[12:8] != outptr[12:8] && !packet_in && !packet_out) ||
              (packet_in && inptr == packet_count && inptr != outptr) ||
              (packet_out && inptr == packet_count /*&& inptr != outptr*/);
assign packet_in_last = packet_in && inptr == packet_count && inptr == outptr && inptr != 0;

assign last_out = outptr[7:0] == 8'hFF ? 1'b1 : 1'b0;
assign last_in  = inptr [7:0] == 8'hFF ? 1'b1 : 1'b0;

endmodule